I personally don't think the Cardassians were any worse than the Romulans or Klingons. They've just been victimized. Sure, they don't have the honor of Klingons or the morals of the Federation, but I think if the Klingons invaded Romulus and nearly conquered it the Romulans would pull off something shady like allying with the Dominion. Hell, they're more treacherous than the Cardassians. Aren't the Cardassians genetically xenophobic because they evolved from reptiles?